Transom height?

[Question]

Hello all

Our first boat and first post

We tried our new 3.1m, air deck with a Yam. 2 stroke, short shaft, 8hp yesterday for the first time. There was no wind (well.. ) and the sea was so smooth that there were even no wrinkles except the ones we made.

As you may guess motor is in break in period so we drive it at about 1/3 throtle max.

And here's the question:
I noticed some (significant?) amount of water coming from where the leg enters the sea and sprays around almost at 90 degrees (well lets say 75 ). It doesn't enter the boat but makes the motor section really wet (not the top maybe but all the sides). This happens at speeds a little above the start point.

Here's a video of the case

http://www.youtube.com/watch?v=te83WK_KjEc

Spray begins after the second half of the video.

As a note, anti-cav. plate is may be a centimeter or two below the range recommended in Yamaha manual (~2.5 centimeters). And leg is almost perpendicular to the hull, manual trim is at the second hole from the transom (there are 6 holes there with this motor as you know).

What do you think, mounting height or trim angle or both?

Thanks in advance

Question

[jyasaki]

I'd guess you've got the motor too deep. Try raising it up a bit (often, the cav plate ends up slightly above the bottom of the hull.) If you start ventilating the prop while running, you've gone too far, and need to drop it back down.

Fixed trim is sort of a tradeoff between pushing the bow up on takeoff, and normal running trim.

Some boat/motor combinations have the splash no matter what, though. A search through this forum should yield some info from others who have gone through the same thing.

jky

[Question]

Thanks for the answer Jyasaki

Indeed, splash problem has resolved today, after I raised the motor about 2cm.

There's another smaller mysterious plate (well... I just don't know its name ) about 10cm above the anti-cavitation one, extending to the opposite side. User manual doesn't mention about this plate and I'm not sure about its purpose, but I guess it's there to reflect water coming from lower section.

After I raised the motor, this second plate becomes almost level with the water surface during cruise now (and maybe a little below when stopped). And it returns each drop coming from the prop back to the sea

Also, the leg section above this upper plate is much thicker and not so much hydrodynamically shaped compared to the section below it, which has sharper edges. Thus I guess this is a limit where the leg shouldn't be submerged more. Please correct me if I'm wrong.

Best

Question

Originally Posted by jyasaki

I'd guess you've got the motor too deep. Try raising it up a bit (often, the cav plate ends up slightly above the bottom of the hull.) If you start ventilating the prop while running, you've gone too far, and need to drop it back down.

Fixed trim is sort of a tradeoff between pushing the bow up on takeoff, and normal running trim.

Some boat/motor combinations have the splash no matter what, though. A search through this forum should yield some info from others who have gone through the same thing.

jky